The field of the invention is static power converters for transforming a.c. power to a controlled d.c. voltage or d.c. current.
Static power converters have numerous applications in industry. They are used to provide controlled power to d.c. motors, and in recent years they are also being employed as a current source or a voltage source supply to adjustable speed a.c. motor drives. Such power supplies are also employed in induction heating equipment, corona treatment apparatus and electrostatic precipitators.
Static power converters have employed analog circuit techniques for many years. Such systems employ operational amplifiers and oscillators which are interconnected to produce firing pulses for the thyristors in the power supply bridge. In a three-phase fully controlled bridge, for example, firing pulses for six thyristers must be produced. The circuitry is complex and is subject to temperature drift and aging.
More recently, digital circuits have been developed to control the firing of the thyristors. In some instances such circuits merely add D/A and A/D converters to basically analog schemes, whereas other digital systems primarily employ discrete digital components. Regardless of the emphasis, these digital schemes require a relatively large number of parts which occupy considerable circuit board space. In addition, each digital converter control circuit must be designed and built for a limited number of end uses. For example, a control circuit for a constant current supply may be substantially different than that for a constant voltage source, and neither circuit may be applicable to an unregulated power supply or to a power supply which is to be controlled by signals indicative of other operating parameters, such as motor speed, armature current, motor flux, etc. As a result, a manufacturer of a wide range of products may require a corresponding large number of different converter control circuits to meet its needs.
One solution to many of these problems is to execute the functions performed by the discrete digital or analog devices with a programmed microprocessor. Examples of such microprocessor-based converter control circuits are disclosed in U.S. Pat. No. 4,347,562 and articles entitled "Microprocessor Controller for a Thyristor Converter With an Improved Power Factor" and "Dedicated-Microprocessor Scheme for Thyristor Phase Control of Multiphase Converters", published in Proceeding of the 1980-IECI Conference, Mar. 1980, and IEE Proceedings, (U.K.) Part B, Vol. 128 No. 2, pp 101-108, 1981. Although such prior microprocessor-based control circuits do reduce the number of components, they still require a substantial number of support components to maintain an adequate response time to changes in converter operating conditions. Even then, the response time of prior microprocessor-based control circuits is not sufficient for some motor control applications.